The present invention relates to baluns, that is transformers capable of coupling a single-ended input to a balanced output and vice versa.
A balun is used to connect together many different types of electronic devices including antennas, transmission lines and electronic circuits when one device is single-ended or unbalanced and the other device is balanced. For example, a receiving system may comprise a balanced antenna which feeds an unbalanced coaxial transmission line which in turn carries signal energy to a receiver having a balanced input. In such a case, a balun is required at the antenna-coax cable interface and at the coax cable-receiver interface for the system to operate properly. Furthermore, RF signal processing components and circuitry inside a receiver often utilize baluns for their interconnection. In addition, at the component or circuit level, a balun may be an integral part of the design to achieve a particular function or level of performance.
In the RF frequency range 1 MHz-2 GHz, baluns are used extensively in many different applications and so a wideband balun which can function over a substantial portion of this frequency range has great utility. It can be used in applications where a wide range of frequencies will be present and alternatively it can provide a single-device solution to many different narrow frequency band problems.
Well-known prior art wideband baluns 20, 22 are shown schematically in FIGS. 1, 2 respectively. Coils 2, 4, 6 and 8 are bifilar windings, where coils 2 and 4 comprise two-wire transmission line 16 and coils 6 and 8 comprise two-wire transmission line 18. Transmission lines 16 and 18 may be wound on individual magnetic cores or on a single, two-hole core. An impedance connected to single-ended terminal 10 will match a balanced impedance connected between balanced terminals 12 and 14. In FIG. 1, transmission line 16 is connected in parallel with transmission line 18 between single-ended terminal 10 and signal ground. Balun 20 is therefore known as a parallel-connected balun and provides a balanced-to-unbalanced impedance matching ratio of 4:1. In FIG. 2, transmission line 16 is connected in series with transmission line 18 between single-ended terminal 10 and signal ground. Balun 22 is therefore known as a series-connected balun and provides a balanced-to-unbalanced impedance matching ratio of 1:1. For an impedance of designated value Z.sub.0 connected to unbalanced terminal 10, the preferred values of characteristic impedance for two-wire transmission lines 16, 18 are 2Z.sub.0 for parallel-connected balun 20 and Z.sub.0 /2 for series-connected balun 22.
There are several known variants of the above-described wideband baluns. U.S. Pat. No. 3,357,023 to Hemmie shows two-wire transmission lines wound on an insulating core rather than a magnetic core. This eliminates magnetic core losses but limits the low-frequency response and bandwidth.
In U.S. Pat. No. 3,327,220 to Podell, a two-wire transmission line passes thru a magnetic core rather than being wound as a coil.
In U.S. Pat. No. 3,846,721 to Fritz et al., two-conductor transmission lines are disposed on a dielectric substrate.
Stray coupling between the two-conductor transmission lines and loss of signal energy in the magnetic core material are important limitations to the electrical performance of prior art wideband baluns. The widely used method of winding both two-wire transmission lines on a single, two-hole core causes troublesome manufacturing difficulty.